Supply voltage detection devices are known in practice, cf. FIGS. 1 and 2. A supply voltage detection device of the type in question is used e.g. when, in a lighting system, the mains voltage fails and when switching to an emergency voltage is to be executed so as to further supply at least a few parts of the lighting system. To this end, the respective mains voltage value is detected and compared with the predetermined reference value. If the mains voltage value is below the reference value, switching to the emergency voltage supply will be effected. It is also possible to switch back from the emergency voltage supply to the mains voltage supply by means of the same electronic control system in response to a re-application of the mains voltage supply and a resultant mains voltage value higher than the predetermined reference value.
The respective predetermined reference value is predetermined e.g. through standards or the like.
FIG. 1 shows an example of a supply voltage detection device 1 known in practice. This supply voltage detection device comprises a rectifier 5 for rectifying the mains voltage supplied by a mains voltage supply 2. The rectified mains voltage is reduced via a voltage divider 13 with resistors 14 and 15, and capacitors 18 and 19 are provided so as to establish an adequately smoothed DC voltage. This voltage is supplied via an output 21 of an electronic control system 3. The applied voltage value is detected by means of this electronic control system and a comparison with the predetermined reference value is carried out accordingly. A load 17 is additionally shown in FIG. 1 in principle.
A disadvantage of the circuit shown according to FIG. 1 is that no galvanic separation exists between the mains voltage supply 2 and the electronic control system 3. Moreover, the load 17 may influence the voltage value detected at the output 21. The capacitors 18 and 19 are normally electrolytic capacitors for smoothing the voltage. These capacitors are additionally necessary, and for precise measurements of the mains voltage value a low-ripple voltage is required, so that the capacitors 18 and 19 have to satisfy the demands in question.
The circuit according to FIG. 2 shows a further example for a supply voltage detection device 1, which is known in practice. This supply voltage detection device is configured substantially analogously to the device according to FIG. 1, except that a transformer 20 is additionally connected between the mains voltage supply 2 and the rectifier 5. This transformer 20 constitutes a galvanic separation. Apart from that, the steps for detecting the mains voltage value and for comparing the latter with the reference value take place like in the case of FIG. 1.
With the exception of the galvanic separation, the disadvantages described in connection with FIG. 1 have to be taken into account also in FIG. 2. In addition, low-power transformers often have a characteristic in the case of which the mains voltage value applied to the output 21 may exhibit high fluctuations at low load variations.
In addition, a different mode of arrangement of the load 17 is shown in FIG. 2, cf. the arrangement between the mains voltage supply 2 and the transformer 20.